1. Field of the Invention
This invention generally relates to the protection of optical discs from illegal copying, and more particularly to optical discs provided with an illegal copying preventing function, recorders for recording signals in the original discs of the optical discs, and readout units for the optical discs.
2. Related Art
In a compact disc (CD), which is a read-only optical disc, signals have been EFM modulated and recorded as pits (or lands) of 3T to 11T long. Here, T is a 1-channel bit period, that is, the length of a pit (or "a land") that regenerates a signal having a predetermined read clock length. Hereinafter in this specification, T will be used with this meaning.
To record the EFM-modulated signal as pits of integer times as long as T within a range of 3T to 11T, the duty cycle of the EFM-modulated signal is compensated. This is signal compensation considering the beam waist of a recording laser beam. That is, when an original disc is subjected to laser beam cutting in accordance with such compensated signal, the pits are formed exactly to a length of integer times as long as T within a range of 3T to 11T. For this compensation, a recorder for original discs is equipped with a duty compensator between an EFM encoder and an E/O modulator.
The illegal copying of optical discs are performed in such a way that, for example, a protective film is removed from a normally manufactured optical disc, a stamper is made by being plated with the exposed aluminum reflection layer used as an electrode, and a copied optical disc is made by injection molding or the like using this illegal stamper.